Calendar of Physics Talks Vienna

The hard‐disk model has exerted outstanding influence on computational physics and statistical mechanics. Decades ago, hard disks were the first system to be studied by Markov-chain Monte Carlo methods and by molecular dynamics. It was in hard disks, through numerical simulations, that a two-dimensional melting transition was first seen to occur even though such systems cannot develop long‐range crystalline order. Analysis of the system was made difficult by the absence of powerful simulation methods.
In recent years, we have developed a number of powerful Monte Carlo algorithms for hard disks and related systems. I will in particular show how the powerful event-chain Monte Carlo algorithm which has allowed us to prove that hard disks melt with a first-order transition from the liquid to the hexatic and a continuous transition from the hexatic to the solid.

Schrödinger’s cat, half-alive, half-dead, existing in its sealed box in this dual state, illustrates the counterintuitive role of measurement in quantum mechanics, but also how the preservation of quantum properties, which dominate at atomic scales (even at room temperature), become less familiar and even absurd for large multi-particle objects. Building computer chips that can take advantage of quantum coherence and entanglement to allow improvements in computation and simulation is much like the challenge of trying to build Schrödinger's cat, using components that operate near absolute zero. This talk will review progress toward this endeavor, and the interesting new physics that arises in fighting decoherence in solid-state systems.

Neutron stars are an excellent laboratory for testing matter under extreme conditions. In particular, a lot of emphasis has been invested in understanding the interior of neutron stars and the equation of state of the different possible phases since its direct consequences for the mass-radius relationship of neutron stars as well as cooling processes. I will review some results for nucleonic, kaonic and hyperonic matter together with superfluidity and their consequences for cooling processes. I will comment on possible constraints not only from neutron stars observations but also from back-to-Earth experiments, such as heavy-ion collisions. I will finally outline future prospects to be tested in neutron stars laboratory.

Janos Bergou (Hunter College of the City University of New York, Department of Physics and Astronomy, USA)

Abstract:

State discrimination is a fundamental measurement primitive in quantum information processing. It is the key element in quantum key distribution, security analyses and probabilistic quantum algorithms. State discrimination deals with the following problem [1]: One is given a quantum system that was prepared in one of N known quantum states, but we don't know which. The task is to identify the state of the system as well as allowed by the laws of quantum mechanics. If the possible states are not mutually orthogonal the problem is highly nontrivial and optimization with respect to some reasonable criteria leads to complex measurement strategies often involving generalized measurements. Finding the optimum measurement strategy is the subject of state discrimination.

As part of the research project "Ultimal" efforts were made to better understand some details in the functioning of Multi-layer insulations (MLIs) as they are long used in space and terrestrial cryogenic applications. It was the goal of this study to pave the way for a replacement of the "empirical black-box approach" that is mostly used in designing MLIs up until now, while attempting to shed some light on discovered unexpected behavior of these materials which can not be explained in a straightforward manner.
This talk will give a summary on the closed study, the experimental efforts that were made and some conclusions that can be drawn from it.

From copper-oxide superconductors to rare‐earth compounds, materials with strong electronic correlations have focused enormous attention over the last two decades. Solid‐state chemistry, new elaboration techniques and improved experimental probes are constantly providing us with examples of novel materials with surprising electronic properties, the latest example being the recent discovery of iron-based high-temperature superconductors.
In this colloquium, I will emphasize that the classic paradigm of solid-state physics, in which electrons form a gas of wave-like quasiparticles, must be seriously revised for strongly correlated materials. Instead, a description accounting for both atomic-like excitations in real‐space and quasiparticle excitations in momentum space is requested. I will review how Dynamical Mean-Field Theory - an approach that has led to significant advances in our understanding of strongly correlated materials - fulfills this goal.
New frontiers are also opening up, which bring together condensed-matter physics and quantum optics. `Artificial materials' made of ultra-cold atoms trapped by laser beams can be engineered with a remarkable level of controllability, and allow for the study of strong-correlation physics in previously unexplored regimes.

The scalar mesons have been one of the most debated issues of
low-energy QCD for decades. The experimental data show the existence of
six scalar isosinglet states - next to the famous sigma meson, there are
five states with the same quantum numbers but higher energies than the
energy of the sigma. If we consider the u and d quarks as degenerate and
work in a theoretical framework that inlcudes strange states as well,
then we can construct two scalar \bar q - q states. Thus constructed
scalars can, of course, describe at most two out of the mentioned six
experimentally known states - but the question is: Which two?
I will present a model with (pseudo)scalar and (axial-)vector mesons that
allows us to answer this question. The scalar (and other) mesons are
important not only in vacuum but also at finite temperatures and densities
as they build order parameters for the chiral transition.

The quantum-mechanical states of large systems are difficult to measure experimentally because of the exponentially large number of variables involved. Yet in systems of indistinguishable bosons, this number is dramatically reduced, and a tomographic reconstruction of the exchange-symmetric density matrix is feasible even for thousands of particles. We present a practical method for experimentally performing this tomography for two-component Bose-Einstein condensates, and extend it to the tomographic determination of correlations between small numbers of particles within a condensate: such correlations can be stable even when the total atom number fluctuates between experimental runs. The tomographic reconstructions of Wigner functions, Glauber-Sudarshan P-representations, and Husimi-Q distributions on the Bloch sphere are compared.
As an application we present the quantum-state tomography of spin-squeezed states of a two-component 87Rb Bose-Einstein condensate. Such states can be used for interferometric metrology beyond the standard quantum limit.